Many downhole tools are actuated by stored mechanical energy sources such as springs or compressed gases. The energy is used to do work on a movable element of the tool, such as a piston or a sliding sleeve. When such tools are operated at great depths, however, the hydrostatic pressure of the wellbore fluid may apply pressures on the moveable element that are comparable to or even greater than the pressures applied by the stored energy. One way to compensate for the large hydrostatic head is to use stiffer springs or higher pressure gas charges to increase the amount of energy stored. That, however, creates a potentially unsafe work environment or may be impossible or impractical to achieve at the surface.
Accordingly, a need exists for an energy storage system that is charged with energy after the system is placed downhole where it is away from personnel and in a high-pressure environment that can help reduce differential pressures. The present invention is directed at providing such a system.